Electrooptical transmission



ELECTROOPTIGAL TRANSMI SS ION Filed April 8, 1930 Il RE QUE N DIY |000020000 30000 40000 VVE/WUR f.' GRAY Patented Aug. 9, 1932 narran staresrArsNr esterna FRANK GRAY, or NRW YORK, N. Y., AssreNoR-.ro BELLTELEPHONE LABORATORIES, :i

INCORPORATED, or NRW Yoan, N. Y., A' CORPORATION OF New .YORK

ELECTRGOPTICAL TRANSMISS'IQN Appiication inea April s,

This invention relates to electro-optical transmission and morelparticularly to apparatus for reproducing an image.

At receiving stations of certain image producing systems heretoforeproposed, image currents are supplied to a glow lamp containing a chargeof pure gas of the helium group, for example, neon. The incomingcurrents modify the intensity of the light radiated by the lamp, whichis viewed through a suitably apertured scanning member which is drivenin synchronism with a scanning member at the transmitting station.

Considerable difliculty has been experienced in producing a well definedimage at receiving stations which operate in the manner described above.In general the image is either blurred, ill-defined or distorted, be-

cause the intensity of the light supplied by theV 2o lamp fails to varyin accordance with, or synchronously with, the variations of the imagecurrents, or the intensity is not reduced to the proper value whenreduced current is supplied to the lamp.

The undesired conditions noted above are due to the fact that the lamp,because of undesired resistance and reactance characteristics,discriminates in favor of current components of certain Jfrequencieslying Within the frequency range used in television, and

certain other signal systems; that it introduces a phase shift of thecurrent components with respect to the corresponding applied Y voltageswhich is not proportional to frequency, and that there is a time lag ofthe light vwith respect to the current flowing through it which isevidenced by the presence of an after-glow. v

An object of the invention i'sto overcome or minimize the disadvantagesnoted above, in a signalling-system.including glow lamps. Another objectof the invention is to improve the quality of the image produced at atelevision receiving station employing glow lamps.` i

Still another object is to4 provide means at a television receivingstation to produce a sharply defined image of the subject scanned at atransmitting station.

l1930. ser'iarNo. 42,4251.

A further `Object is to Vimprove the fre,- quenoy characteristics ofglowlamps;

An additional obj ect is to prevent light lag effects and particularlyafter-glow in the operation of a glow discharge lamp.

In one preferred embodiment of the invention, the above noted objectsmay be accomplished by providing a receiver comprising an amplifier forsupplying image currents to a glow lamp having a gas charo'e comprising60 amixture of inert gas and a small percentage of active gas.

Because of the use of the active gas, the resistance of the lamp ismaintained substantially constant over the television frequency range,its reactance varies approximately linearly with frequency, Vand itsimpedance is lower than the impedance of a lamp charged with the samepractically pure inert gas. The active gas also operates tosubstantially eliminate the light lag with change of current iowingthrough the lamp.

rThe present invention also contemplates including the glow lamp in anamplifier output circuit having an impedance which is con-v stant overthe television range and which is high compared with the impedance Vofthe lamp, whereby the effect of the `impedance variations of thelamp,uponr the amplitudes of the different frequency com onents of the 8oimage current wave, is rendere substantially negligible. n V

A more detailed description ofthe invention Vfollows and is illustratedin tliefaccompanying drawing.

1 s Fig. 11s a schematic diagram of a receiving circuit embodying theinvention and Figs.2 and 3 illustrate curves, used to explain theinvention. i Y Referring tOFig. l, there is shown a cil- 9'0 cuit l forapplying the incoming image currents to theinput circuit of anamplifier.

Obviously the circuit 1, may be the termina-v tion of a line over whichthe image current-s are transmitted or the output circuit of a hiOhfrequency terminal apparatus, vif the image currents are'transmitted as.modulailziois of a carrier wave over a lineor radio The image currentsare impressed upon the 1b@ thev changing-amplitudes of .the amplifiedim# input-electrodes of the space discharge tube amplifier 2, the outputcircuit Q of which includes a glow discharge lamp 3.

The input circuit of the amplifier includes a battery 4, for impressinga biasing potential upon its grid 5, and a resistance 6, for im-Vproving the operation of the tube 2, aS an amplifier. Battery 7,supplies space current to the anode-cathode path of the tube 2, through'Vthe discl'iar,e` path of the 74Vglow lamp 3. Y v

The intensity of the light produced by the lamp 8, is caused to varyin'accordance with age current supplied by the tube 2.

`Av v.In television systemsvthe glow area vof the lamprS is viewedthrough the apertures of a scanning member, which is driven -in syn--chronism with a similar member at the transmitting station. Y Thescanning members areoperatedat such a rate of speed that an image of thesubject, scanned at the transmitter, is completely repeated atrthereceiving station in a period corresponding to the persistance ofvision, i. e., from le to ,La of 'f va second. The frequency range ofthe image currents is fromV very near zerol :to from 10,000 to50,000cycles .per second, depending i upon 'the size of the image, thescanning peri- .,od and the vresolution desired. Examples of `televisionsystems in which the circuit of Fig. 1 may be employed, or in which theglow lamps .of this invention may be substituted forthe ones shown, arethose-disclosed in application of F. Gray Serial No. 227,649

led October V21 1927 and application of J. T.F.'-Iorton SerialNo.230,418 filed November 2, 1927. y Y The glow lamp 3, is provided with agas charge comprising an inert gas and a small quantity of active gas.For example, the Charge may comprise 99 to 90 per cent neon *Y and 1 toA10 per cent hydrogen.

With regard to the alternatingcurrent componentsl which flow through theglow lamp j8,the circuit of Fig. l'may be considered as though theamplier tube 2, were a source for supplying alternating current at Y anE. M. F.=E and had anA interna-l resist-- ance-1K0. That is, theinternal output resistance of the tube-2, is equal to Ro'ohms.

` The lamp 8, maybe regarded" as an impedance Z, comprising a resistanceR, and a reactance X.

' The absolute value of theimpedance Z Vx/RZ-l-Xz.

Interms of the voltage `drop"V across thev lamp, the current'through itat any frequenand =thephase angle between current and voltage is =arctanalf R Y In the operation of the circuit of Fig. 1, current of anyfrequency flowing through the lamp 2, is

Y (R R02 jl- X 2 and the phase angle between the current and E. M. F.is:

1 X @FMP im For small values of i X Y I RQ'p-l-R kthe phase .angle isapproximately .@ZR-PR j' Referring now to Fig. V2 there are shown vtwosets of curves. Oneset, A, B and C relates to a discharge lamp chargedwith pure neon, and the other set, D, E and F to the same lamp, providedwith a gas chargecomprising'90% neon and 10% hydrogen by volume and at apressureY of a few millimeters of mercury.

the pure neon lamp varies from about 1,800@

at approximately zero frequency `to about 5,300@ at 40,000 cycles.

Curve B indicates that the reactance X of the pure neon lamp `variesfrom about 400w at substantially zero frequency to about 5,500@ at40,000 cycles.

Curve C, which relates to a 3,000w circuit including the pure vneonlamp, indicates that the total impedance of the circuit varies from4,800w at'approximately zero frel stantially Zero frequency `to 4,700@at 40,000

cycles. Y y

Fig. Bincludes curves .Grand H which indijcate the Vphase V'angle vbetween the applied i F. and current,v over the Zero to 40,000

Ycycle range, for the itwo lamps referred to above. Y

As shown by curve G thefphase shift inthe 'pure neon lamp varies from 0%for direct current to about-42% at 22,500 cycles and 35% at 40,000cycles. Y V` According to curve A the resistance R of For the lampprovided with a gas charge including 10% hydrogen, curve H Vindicatesthat the phase shift of the applied current Vwith respect to the appliedE. M. F. varied from zero, for direct current, to 241/2% at 35,000cycles and 23% at 40,000 cycles.

Thus, the presence of the active gas lowers the impedance of the lampand makes Vit a smaller part'ofV the total Vimpedance of any circuit inwhich it is used, and the resistance `of the lamp is made more nearlyconstant over Vthe television range. Both of these factors result in amore uniform current response at the various frequencies. By loweringthe reactance of the lamp, the active gas renders the phase shift morenearly proportional to frequency and, in a circuit of reasonably highimpedance, gives a phase shift which is practically proportional to thefrequency over the complete range. The curves show that thisproportionality holds up to about 35,000 cycles for the improved lampsherein described when included ina 3,000wcircuit.

While many advantages result from the use of a small percentage ofactivegas in a charge of inert gas, it will be apparent that to secure thebest results, it is necessary to use the lamp in a circuit having animpedance which is high compared with the impedance of the lamp.

Within wide limits, the impedance of the Vlamp is improved as thepercentage of active gas is increased. However, since the presence ofthe active gas serves to reduce the brightness of the light produced,there is a maxiinum limit which should not be exceeded.

Under present conditions in television, not more than 5% of hydrogenshould bevused. As a matter of fact, the advantages of this in ventionare completely realized in a neon lamp having a hydrogen content of l or2%.

In addition to improving the impedance characteristics of the lamp, theactive gas effects a result of major importance in electrootpicalsystems in that it prevents after-glow, i. e. the production of a glowwithin the lamp after the flow of image current through the `lamp hasceased'. The production of afterglow may be explained lon the theorythat, when an electron collides with an inert gas atom, the electronbounces i away from the atom with practically no loss of energy. ln apure inert gas, electrons can thus lose their energy only very slowlyandare consequently slow in recombining with positive ions. Thisresidual ionization, or time lag in change of ionization, causesafter-glow which makes the lamp act somewhat as an inductive rea'ctancein an alternatingcurrent circuit and-causes the image to be blurred.

However, when an electron collides with an active gas molecule itbounces awaytherefrom with very little of its initial energy. Thepresence of a small percentage of active gas in an inert gas, therefore,causes the electrons to lose theirfenergy more rapidly than in a pureinert gas and causes them to recombine more quickly with the gasmolecules, whereby after-glow is substantially obviated.

Quite independent of the above or any theory as to the cause ofafter-glow in the operation of glow lamps includuing a pure inert gascharge, it has been determinedthat this undesirable condition can beobviated by adding a small percentage of active gas to an inert gascharge.

There is evidence also that a lag in ionization occurs when current isapplied or increased, and that this effect is reducedin the improvedlamp herein disclosed.`

The present invention provides a glow lamp having an inert gas charge towhich active gas is added for the purpose of determining and controllingthe response of the lamp when it is producing light i. e., underoperating conditions; whereas heretofore active gas was introduced intoa glowV lamp for the sole purpose of lowering its starting or break downvoltage. l

lihile a definite circuit arrangement has been described and certainspeciic details have been given, it is to be understood that this wasdone merely for the purpose of completely and clearly disclosing theprinciples and nature of the invention. lts novel features will bedefined in the attached claims.`

Obviousy the present invention is applicable to sound recording, inwhich the distortion produced by the ordinary glow lamps including apure inert gas charge, is applicable. The invention is of particularvalue for such use when the frequency range is wide, as in high qualityrecording.

l/Vhat is claimed is:

prising a circuit,means for producing` image itu currents in saidcircuit, and included in said circuit a glow discharge lamp having meansfor maintaining its resistance substantially constant over the frequencyrange involved and its reactance varying approximately linearly withfrequency.

2. An electro-optical receiver lcomprising a glow lamp, a circuitincluding said lamp and having high impedance compared therewith, andmeans for maintaining substantially constant the resistance of said lampover the frequency range involved.

3. A television receivercomprising a glow lamp, means for repeatingimage currents in a circuit including said lamp and having highimpedance compared therewith, and means Lfor causing the reactance ofsaid lamp to vary proportionally with frequency over the frequency rangeinvolved.

4. A television receiver comprising a glow lamp, means for producingimage currents in a circuit including said lamp and having highimpedance compared therewith, said elow lamp including a chargecomprisingan inert lamp, means for repeating image currents in a circuitwhich includes said lamp and has high impedance compared therewith, andmeans for causing said lamp to have a resistance which is substantiallyconstant, a react-k anc-e variable substantially linearly with frequencyand a phase shift proportional to frequency for currents in thefrequency range involved. f

6. An electro-optical system comprising a glow lamp, means for supplying.to said .lamp current extending over a wide frequency range, and meansfor causing the reactance of said lamp to vary substantially linearlywith frequency for currents within said range. Y

7. An electro-opticalsystem comprising a glow lamp, means for supplyingto said lamp current extending over a frequency' range includingfrequencies above 10,000 cycles per second, and means for causing thereactance of said lamp to vary substantially linearly with frequency forthe current supplied by said means.

8. An electropptical system comprising a glow lamp, means for supplyingto said lamp current of a frequency range from below Y10,000 cycles persecond'to at least 20,000 cycles per second, and means for causing thereactance of said lamp'to vary substantially linearly with frequency forthe current supplied by said means. y

9. An electro-optical system including a glow lamp, and means forcausing the reactance of said lamp to vary linearly with frequencywhensupplied with currenthaving frequencies extending over a wide range.

10. An electro-optical system including a. glow lamp, and means forcausing the reactance of said lamp to produce a phase shift proportionalto frequency for current supplied thereto, having frequencies extendingover a wide range.

11. A circuit, a glow discharge Vlamp in said circuit and means forimpressing upon said circuit signaling potentials having a. wide rangeof frequency components for setting up light variations corresponding tosaid signaling potentials, said lamp having a gaseous filling comprisingan inert gas and a small percentage of active gas, se that the combinedimpedance ofvsaid circuiti and said lamp is substantially constant oversaid range of frequencies.

12. A television system comprising a space discharge amplier, means forimpressing an image current band on said amplifier, a glow dischargelamp included in the output circuit of said amplifier, said lamp'havingagaseous lilling of neon and less than 10% of hydrogen,

Ithe combined impedance of saidoutput circuit and lamp beingsubstantially constant over said image band range.

' lf3.V Thecombination with4 a glow lamp-hav-` ing argascharge' of inertgas and a small T70 amount of active gas, and means vforimpressingupon-a-circuitincludingsaid lamp Ya variable' electromotive Vforcehaving freqeuncy components extending over awide frequency range,whereby the Vimpedance of said com- 4 bined circuit andlamp issubstantially constant and the response of said lamp'isv substantiallyproportional at each instant to the Aamplitude of the'impressed'electr'omotive 8,0

taining a gaseous'filling comprising inert vgas so and an active gas,the proportion of the latter by volume Ybeing of the order of 10% orless, means for impressing uponsaid lamp variable potentials havingfrequency components extending over a frequency range of at 9,5

least live thousand cycles per second.

16. The combination lof a. glow discharge lamp containing inert andYactive gas, theportion of active gas by volume being of the order of10%l or less, means for impressing a steady biasing potential upon saidlamp and means for impressing upon said lamp variable potentials havingfrequency components extending over a range of .at least twenty thousandcycles per'second.

17. The combination with 'a'glow discharge lamp havinga gaseous fillingcomprising inert gas andhydrogen, the portionof hydrogen by volume being10% or less, and means for impressing vupon said lamp variable.potentials comprising a wide range of frequency components.

18. The combin ation with a glow discharge lamp containing a gaseousfilling comprising inert gas and hydrogen, the vproportion of the latterby volume being 5% or less, and means for impressing upon said lampvariable potentialscomprising a wide range of frequency components.

19. Means for producing atelevision image .1,20

comprising a glow discharge'lamp having a gaseous filling of inert gasand active gas, the proportion by volume of the Vlatter being of thevorder of 10% orl'ess,Y and means yfor impressing upon said lamp imageypotentials having frequency components extending 4over a range of twentythousand cycles per second or more. Y

ln witness whereof, I'hereunto' subscribe myname this 7 th day of April,193.0. l i

' f VFl'tANK GRAY.

